1. Field of the Invention
The present invention relates to a numerical controller for controlling a multi-axis machine having at least three linear axes and three rotary axes. More particularly, the invention relates to a numerical controller configured to perform compensations corresponding to table linear-axis translation and rotation errors dependent on a linear axis position for table drive, table rotary-axis translation and rotation errors dependent on a rotary axis position for table drive, tool head linear-axis translation and rotation errors dependent on a linear axis position for tool head drive, and tool head rotary-axis translation and rotation errors dependent on a rotary axis position for tool head drive, thereby moving a tool center point to an error-free position and moving the tool posture (orientation) to an error-free tool posture and ensures control for high-precision machining.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2009-151756 (Also published as US2009/0140684A1, hereinafter referred to as the patent document) discloses a technique for a numerical controller for controlling a five-axis machine having three linear axes and two rotary axes. According to this technique, a linear-axis coordinate space and a rotary-axis coordinate space are each divided into lattice-like regions, lattice point compensation vectors are stored at lattice points, axis-dependent translation and rotation compensation amounts are calculated from linear and rotary axis positions, based on the lattice point compensation vectors, and a tool center point is moved to an error-free position by compensating the linear axis position.
According to the patent document described above, only the linear axis position is compensated. Thus, although the tool center point is moved to the error-free position, the tool posture (orientation) remains erroneous.
If the tool center point is used for machining, the above-described prior art compensation involves no problem. However, if a side face of a tool is used for machining or in the case of boring in which machining is directed toward the tool, the tool posture (orientation) is an important factor, and simple compensation of the tool center point position to an error-free position is not enough. Thus, according to the prior art disclosed in the above patent document, high-precision machining cannot be achieved if there is any error attributable to a machine system during the boring or machining with the side face of the tool.
Further, the object of the technique disclosed in the above-described patent document is the five-axis machine having three linear axes and two rotary axes. Five-axis machines can be roughly classified into three types, a tool-head-rotation type, table-rotation type, and mixed type (in which both a tool head and a table are rotatable). In the five-axis machines of the table-rotation type and mixed type, the rotary axes cannot always be controlled so as to compensate an error, if any, in the tool posture (orientation) relative to a workpiece.
In the above-described patent document, moreover, the main possible errors are described as including (1) a linear-axis-dependent translation error that depends on the linear axis position, (2) a rotary-axis-dependent translation error that depends on the rotary axis position, (3) a linear-axis-dependent rotation error that depends on the linear axis position, and (4) a rotary-axis-dependent rotation error that depends on the rotary axis position.
However, the main errors derive from a moving body. In general, a moving body in a machine tool is a tool head or a table. Normally, a rotary moving body lies on a linear moving body. Therefore, an error in the rotary tool head depends on the position of a rotary axis about which the tool head is rotated and that of a linear axis on which the tool head lies. On the other hand, an error in the rotary table depends on the position of a rotary axis about which the table is rotated and that of a linear axis on which the table lies. Thus, the four error categories disclosed in the above-described patent document are given in a simplified form, and the errors should be originally classified into eight categories, as shown in FIG. 1.